Liquid crystal device including an alignment grating with breaks therein

ABSTRACT

This invention relates to a liquid crystal device wherein the surface profile of a surface alignment grating stabilizes at least one stable state. The invention involves the introduction or breaks ( 34, 36, 38 ) or discontinuities into the grating ( 10 ) to divide the grating into a plurality of groove segments. The breaks are discontinuities in the grating in the groove direction, the grating having substantially the same groove direction on each side of the break. The introduction of breaks prevent free movement of defects along the groove of the grating and therefore help to stabilize the desired liquid crystal configuration, either a Defect state or a Continuous state. Suitable breaks involve gaps ( 34 ) in the groove ridges ( 30 ), necks ( 36 ) between the groove ridges ( 30 ) and slips or relative displacement ( 38 ) of the grating.

This application is the US national phase of international applicationPCT/GB04/000519, filed 9 Feb. 2004, which designated the U.S. and claimspriority of EP 03250808.7, filed 7 Feb. 2003 and US ProvisionalApplication No. 60/456,526 filed 24 Mar. 2003, the entire contents ofeach of which are hereby incorporated by reference.

This invention relates to a liquid crystal device having a regularsurface alignment grating stabilising a particular liquid crystalconfiguration and especially to a multistable device wherein at leastone stable state is a Defect state and in particular to such a liquidcrystal device wherein the regular surface alignment grating has breaks,or discontinuities, therein.

Bistable or multistable liquid crystal devices are commonly used asdisplays for displaying information stored in an electronic form. Thedevices are generally used in a pixelated form and either directlydriven, matrix or actively addressed. Other uses for such devices are aslight modulators, optical microwave or infrared shutters. The device mayuse bistability only occasionally or only partially. A typical deviceincludes at least two electrode structures, means for applyingappropriate signals and means of discriminating the two states, such asdyes, polarisers, reflectors, absorbers and illumination sources.

U.S. Pat. No. 5,357,358 describes a bistable liquid crystal devicewherein both internal surfaces of the cell have a treatment giving riseto various preferred alignment directions of the liquid crystal materialadjacent the surface. Careful arrangement of the surface alignmentdirections on opposing plates can lead to two stable states for theliquid crystal material at different azimuthal angles. U.S. Pat. No.5,796,459 describes another surface treatment that can be used to givetwo stable states with different azimuthal orientation of the liquidcrystal director.

Zenithally bistable liquid crystal devices are also known. U.S. Pat. No.6,249,332 describes a liquid crystal device wherein at least oneinternal surface has a profile which allows the liquid crystal materialto be in one of two stable states, a Continuous state or a Defect state,the two states having the same azimuthal orientation of the liquidcrystal director but different zenithal orientations.

International Patent Application WO02/08825 describes how the carefuldesign of surface profile can lead to Defect states with the defectsforming close to or at predetermined features of the surface tostabilise certain configurations. In this way more than one stable statecan be achieved.

WO01/40853 describes another bistable device having a surface designedto give local zenithal bistability. In this device however the profilechanges over length scales of less than 15 micrometers. These changesare designed to vary the director orientations within each pixelcreating micro-domains that can, for example, give scattering ofincident light.

Bistable devices are also known where the combination of two suitablemonostable surfaces can lead to bistability, for example as described inU.S. Pat. No. 4,239,345, U.S. Pat. No. 6,327,017 or U.S. Pat. No.4,333,708.

In general bistable or multistable devices work by ensuring that theenergy associated with the liquid crystal configuration is locallyminimised at each stable state and that an energy barrier exists betweenthe various stable states. In defect stabilised multistable liquidcrystal devices the cell has at least one state where defects haveformed. Whilst the defect represents a disclination of the liquidcrystal director field and therefore some energy is contained in thedistortion, the formation of defects in certain areas can result in theenergy being minimised as compared to a different stable configurationwithout defects present or a configuration having a differentarrangement of defects. This minimum may be either the global minimum ofthe system, or a local minimum separated from other minima by an energybarrier.

In defect stabilised devices where the surface profile provides thestabilisation of the defects the surface relief structure has a profilein one direction that has at least one concave edge and at least oneconvex edge. These edges act to stabilise the defects or disclinationsof strength ±½. The structure is repeated across the surface to give twoor more stable or meta-stable states. This is surface multistability,i.e. the stable states are produced regardless of the treatment on theother surface, although obviously the treatment on the other surfacewould effect the overall configuration. The repetition may be periodicor aperiodic, but with a maximum and minimum separation of the reliefstructures. Such surfaces are used in U.S. Pat. No. 6,249,332 andWO01/40853. Common to all prior art surface defect stabilised devices isthat the at least once concave edge and at least one convex edge isparallel to the plane of the surface.

The energy barrier between the various stable states should be highenough to ensure that the correct state is selected and maintainedacross a range of operating conditions. It will be noted that in matrixaddressed displays one pixel may have a field applied before or after itis actually being addressed to cause correct latching. Correct latchingneeds to be maintained across a range of operating conditions.Temperature, mechanical stress and field inhomogenities all play a partin the various latching thresholds. Consistency of latching is obviouslydesirable. However the energy barrier should also be low enough that thestates can be selected by application of the appropriate fields.Multistable devices are often used with portable, battery drivenappliances where power consumption is an issue and minimal voltage orpower to latch would be advantageous.

Further it will be appreciated that the liquid crystal material isgenerally a continuous layer but may be divided into pixels which areseparately addressable. Thus neighbouring pixels, or the liquid crystalmaterial in the inter-pixel gap, may be latched into different stablestates. The liquid crystal material at the interface is thereforesubject to the elastic forces from the adjacent material which may causepartial “grow-back” or creep of the wrong, i.e. undesired, state. Somedevices even have different domains within a pixel, i.e. sub-pixelshaving a different surface profile to other sub-pixels, to allow forlatching at different thresholds to give greyscale. Again the problem ofgrow-back may be encountered.

Even when a device is intended to be monostable in operation it mayrequire a particular liquid crystal stable configuration to functioncorrectly. Change to the operating conditions could result in a changeof the relative energies of states leading to growth of an unwantedstate and incorrect operation.

It is therefore an object of the present invention to provide animproved multistable surface treatment for a liquid crystal device andto provide an improved multistable liquid crystal device.

Thus according to the present invention there is provided a liquidcrystal device comprising a layer of liquid crystal material disposedbetween two cell walls, at least one region on the internal surface ofat least one cell wall comprising a surface alignment grating having asingle groove direction characterised in that the surface alignmentgrating comprises a plurality of breaks along the groove direction.

The surface alignment grating comprises a series of grooves, the profileof which is sufficient to cause the liquid crystal material adjacent thegrating to adopt a particular stable configuration. In some embodimentsthe surface alignment grating may comprise a multistable surfacealignment grating the profile of which is sufficient to cause the liquidcrystal material adjacent the grating to adopt two or more stableconfigurations, one of which will be a Defect state. In any case thegrating may be periodic, in that a certain profile variation is repeatedacross the region, or aperiodic in that the surface profile has anirregular variation. The grating has substantially a single groovedirection however in that the direction of all the grooves in the regionis substantially parallel. The groove direction is the direction inwhich the groove extends. In a prior art multistable device using agrating structure without breaks, such as described in U.S. Pat. No.6,249,332, it can be seen that the surface profile varies across thecell wall in a plane orthogonal to the groove direction but is constantin a plane which is parallel to both the groove direction and the normalto the cell wall.

As described in U.S. Pat. No. 6,249,332 and in WO02/08825 the effect ofthe surface profile is to allow at least one stable liquid crystalDefect state in which the formation of ±½ defects stabilise the liquidcrystal material. Another stable state is the Continuous state where nodefects are present. By careful design of the surface profile the energyassociated with each state can be a local minimum with an energy barrierassociated with moving from one stable state to another. Therefore theliquid crystal material can adopt any of the stable states and can belatched between the various states on supply of an appropriate impetus,usually an electric field pulse of correct polarity.

As described in WO02/08825 the sites where defects form in the Defectstate are determined by the surface profile. In the plane orthogonal tothe groove direction the presence of concave and convex surfacecurvature favours the formation of defects in the vicinity. Usually,this surface curvature leads to concave and convex edges. The energy ofthe system is therefore a local minimum when defects are formed and solocated. This acts to keep the defects pinned in the plane orthogonal tothe groove direction. Along the groove direction defect lines extend inthe vicinity of the edges.

As explained above the devices are formed as a plurality of pixels whichcan be separately addressed. There may even be sub-pixel areas within asingle addressable area with different latching characteristics to allowfor greyscale. Therefore the situation will arise where the liquidcrystal device is in a Defect state in one area that is adjacent an areain the Continuous state. Therefore there will be a short transitionregion where the liquid crystal material goes from a Defect state to theContinuous state. In this region the position of the defects, i.e. the+½ and −½ defect pair, relative to the concave and convex edgesgradually alters across the transition region in the groove direction,the defects approaching one another as one gets closer to the region ofContinuous state, until the defects meet and annihilate at anannihilation point. Each +½ and −½ defect pair may form a defect loop,separated by at least two annihilation points.

In the transition region the elastic forces of the adjacent liquidcrystal material acts upon the material in the transition region. Thebulk material in the Defect state tends to act upon the material in thetransition region to induce it to adopt the Defect state whereas thematerial in the Continuous state tends to act to form the Continuousstate. In the absence of an addressing field the relative energies ofthe two states will in large part determine what happens in thetransition region. In the prior art there is no pinning of theannihilation point in the groove direction. Therefore if one state ismore energetically favourable than the other it can have a greatereffect on the transition region and start to grow into the other state.This effect, known as grow-back, can therefore lead to the pixel orsub-pixel assuming the incorrect or undesired state. Even where the twostates are energetically equal mechanical disturbance, or temperaturechange could change the relative energies of the two states, therebydisrupting the equilibrium and start grow-back. Alternatively, the shapeof the grating may vary. This variation may be deliberate or may beaccidental due to uncertainties in the process conditions. Suchvariation may cause one state to be favoured over another, therebyallowing unimpeded grow-back into this state in situations where it isundesirable.

The discussion above has concentrated on multistable surface alignments.However certain liquid crystal devices are intended for monostableoperation, i.e. they should always relax to a desired state when notbeing addressed, but require a particular liquid crystal alignment forthe correct optical properties. As mentioned above however mechanicaldisturbance or change in temperature may alter the relative energies ofcertain configurations and another, undesired, state may start to becomemore energetically favourable. Further as mentioned, process variationsin manufacture of the grating may mean that the grating properties varyslightly across the device. Therefore grow-back could be an issue evenin some circumstances even in monostable devices

Further, for both monostable and multistable devices, the nature of theliquid crystal material means that there will be free ions in the liquidcrystal material. Where the liquid crystal material is addressed by anelectric field of a particular polarity application of the field willtend to cause migration of the ions. When the addressing field isremoved however the concentration of positive ions towards one surfaceand negative ions toward the other will create a temporary field of theopposite polarity until equilibrium is reached again. This ionic field,being of the opposite polarity to the addressing field, will act uponthe liquid crystal material in such a way as to promote to latching tothe incorrect state. This is a well-known effect in liquid crystaldevices and the device is designed such that the latching characteristicare such that the reverse ionic field is insufficient to cause latching.However the presence of the field can disrupt the equilibrium betweenstates at a transition region so as to start a grow-back process.

Due to the nature of the surface alignment grating in most prior artdevices, for example surface roughness, there will be a certain degreeof natural pinning of the annihilation point, and therefore resistanceto grow-back, due to the physical characteristics of the materials ofthe surface but this will tend to be weak and easily overcome. Thereforethe prior art devices are susceptible to the undesired state growingfrom one pixel or sub-pixel (or the inter-pixel gap) to another pixel orsub-pixel.

Note that in some terminology the term pixel can mean more than oneseparately addressable area. For instance a 64 by 64 pixel colourdisplay may actually have three separate addressable areas in what maybe termed a pixel, each being a different colour. Similarly, a pixel mayalso be sub-divided into separately addressable areas to creategreyscale. However for the purposes of this specification the term pixelshall mean an area of the device which is separately addressable.

The present invention therefore deliberately introduces breaks into thegrating in the groove direction. The term break means a discontinuity inthe grating in the groove direction located between two groove sections,i.e. a break in the smooth nature of the grating in the groovedirection. Conveniently, the groove direction will remain insubstantially the same direction either side of the break. The break mayhave no physical dimension in the direction parallel to the groovedirection, or it may have a finite width. The width of the break in thegroove direction will be limited to 5 μm, preferably less than 1 μm andmost preferably less than 0.25 μm. As will be explained in more detailthe breaks may comprise protrusions in the troughs of the groove or gapsin the peaks or both. Additionally or alternatively the breaks couldcomprise a slip or displacement of the groove with components in adirection perpendicular to the groove direction. For example, the slipmay occur in the plane normal to the groove direction and parallel tothe plane of the cell. Other slip planes exist, including verticalslips, or planes that are not normal to the groove direction.

The breaks therefore effectively divide the grating into a number ofgroove segments and a break is something located between two groovesegments having a single common groove direction. Each groove segmentwill have a surface profile that is constant along the groove directionbut which varies along a direction perpendicular thereto. However at thebreak at least part of the surface profile will vary along the groovedirection resulting in concave and/or convex edges in a plane containingthe cell normal and the groove direction. The breaks effectively createan energy barrier to be overcome in moving any defects away therefromcreating a degree of pinning of the defects. This energy barrier meansthat the defects are not free to move in the groove direction which canprevent grow-back of an incorrect state. Although grow-back can occuralong a groove segment the presence of a break will tend to pin thedefects and therefore the annihilation point in the vicinity of thebreak. Therefore grow-back will not progress beyond the break.

Further the breaks, being areas which energetically favour the formationof defects, tend to act as nucleation sites for the formation ofdefects. This can lead to lower voltages being required for latching aswill be explained in more detail later. There are also advantages interms of operating window. With breaks acting as reliable nucleationsites the liquid crystal device has a more reliable and controllableresponse at a range of operating temperatures. It also allows a greaterchoice in the design of surface profile leading to multistability.

Generally the breaks separate grating segments wherein the gratingsegments have substantially the same surface profile on either side ofthe break in the groove direction. In other words the break is adiscontinuity in what would otherwise be a continuous uninterruptedgroove extending in the groove direction. The skilled person willappreciate though that inaccuracies in manufacturing processes may meanthat there are small differences in the shape on either side of thebreak. Also with slips or displacements the alignment of the gratingsegments is obviously out of phase. What is intended however is that thegrating either side of the break is intended to give the same stateswith the same latching characteristics.

Breaks may also be formed from changes of shape to the grating thatoccur suddenly at a dislocation plane. For example, such breaks mayinvolve a change of the mark-to-space ratio of the grating groove toridge, or a change in shape from sawtooth to trapezoidal gratings.

It should be noted that if the pitch of the grating were to vary acrossa slip break the difference in pitches will result in that in some areasa groove on one side of a break line will encounter a trough on theother side but further along the break line the grooves may align so asto effectively result in no break. However a change in the mark to spaceration, i.e. ratio of width of peak to trough, will not necessarilyalter the pitch and a line of breaks across a grating could be achieved.Also it is noted that if the grating shape changes across a break thetwo areas could have different latching characteristic which couldpotentially mean that such a break does not prevent grow-back. Thepresence of such breaks could still aid in formation of certain stateshowever and where the latching characteristic are the same but differentDefect states are formed this may not cause a problem.

Preferably the width of the break in the groove direction is less thanless than 0.5 μm or preferably less than 0.25 μm. As will be describedthe area of the break between the groove segments may not have the rightalignment conditions and therefore the liquid crystal material in thisarea may be in the incorrect state. Therefore in order to minimise thepossible optical effects the width of the break may be minimised.

Conveniently the breaks comprise surface features having a profile in aplane containing the grating normal and the groove direction which hasat least one concave and at least one convex edge.

The grating normal is the normal to the grating as a whole, not thenormal to the locally varying surface. As mentioned in the planecontaining the grating normal and the groove direction the profile ofthe prior art surface alignment gratings is constant. The breaks howeverintroduce concave and convex edges in this plane. These concave andconvex edges act to pin defects in the vicinity thereof and thus providepinning of the defect lines in the groove direction. The introduction ofsuch features therefore introduces corners into the surface profile.

Preferably the radius of curvature of the at least one convex or atleast one concave edge is less than 2 μm, preferably less than 1 μm andoften less than 0.4 μm. Generally the smaller the radius of curvaturethe greater the pinning effect of the edges.

Conveniently the at least one convex and at least one concave edge arejoined by a surface, at least part of which is substantially parallel tothe grating normal.

One convenient form of break comprises an area where the profile of agroove trough is raised. Raising the contour of a groove trough providesa change in profile in the groove direction in the trough. This can actto pin defects at the edges of the raised area. Conveniently the raisedarea or protrusion is at the same level as the peak. In other words atthe break the profile in the groove direction rises from the groovetrough to the same level as the peak then drops back down to the levelof the trough. In the orthogonal direction the profile will actuallystay level across the break from the peak of the groove. Of course thenext groove along may or may not have a break at the same place and so abreak between two groove segments for one groove may be adjacent acontinuous part of another groove segment, or even a continuous groove.It should be noted here that the raised area here represents adiscontinuity in the profile in the groove direction in the trough butthe profile in the groove direction across the peak is continuous. Thus,taking a groove to comprise a peak and a trough, a break in the gratingcan be a discontinuity in either or both of the peak or trough.

As mentioned such breaks, which may be termed necks, will tend to pindefects in the groove direction along the edges of the rise on eitherside of the neck. Where a Defect state is on one side of the neck andContinuous state on the other the defects present in the defect linewill be guided by the raised area to an annihilation point which will bepinned. The breaks therefore act to pin the defect loop formed by thelines of defect running through the liquid crystal material, either ator close to the surface. This will happen even when the liquid crystalmaterial is in Defect state on both sides of the neck, i.e. on each sidethe defects will be guided to an annihilation point. This means that theliquid crystal material adjacent the area at the top of the rise will bein the continuous state. Hence the presence of breaks of this naturewill inherently mean that there may always be a slight amount ofContinuous state present in a Defect state which will reduce contrastslightly. Therefore it may be advantageous to minimise the area of thetop of the rise, or neck.

Additionally or alternatively the breaks may comprise areas where theprofile of a groove peak is lowered, preferably to the level of thegroove trough. The breaks effectively comprise gaps in the peaks. Theedges formed where the peak of one groove segment ends again act to pindefect loops in the defect state, in a similar manner described abovewith regard to necks. Again therefore the area of the gap will always bein the continuous state. It should be observed that the precedingdescription is the case for a homeotropic surface condition. If theliquid crystal aligns in a planar direction with at least a componentperpendicular to the groove direction, then the Defect state andContinuous state conditions are reversed. For example, in such a planaraligned device, a gap may lead to a small island of pinned Defect state.

A further type of break comprises areas where the grooves, or groovesegments, on either side of a break are displaced relative to each otherin a direction having a component perpendicular to the groove direction.In other word the break is an area where the groove undergoes a slip.One can think of this type of break being formed by taking a continuousgrating and dividing it in two along a line which is not parallel to thegroove direction. One part of the divided grating is then displacedslightly relative to the other. This will create the situation where thepeak of the groove of one grating segment does not align with the peakof the next groove segment. This will inherently create a discontinuityin the groove direction at the interface. Of course the actual gratingis not necessarily produced in this fashion. Methods for forming gratingstructures are well known in the art, for instance photolithographyusing appropriate masks. A mask could readily be designed with slips inthe grating and used when exposing a photoresist material.

Conveniently the two groove segments are displaced relative to eachother in a direction which is substantially perpendicular to the groovedirection although other directions are possible provided that thedirection has a component perpendicular to the groove direction.

This type of break will, as mentioned, create a discontinuity in theprofile in the groove direction at the interface between groovesegments. Depending upon the degree of relative displacement thisdiscontinuity can pin the defects present in the defect state in variousways. Take the simple situation where the grating is periodic and therepeat unit consists simply of a peak and a trough of roughly equalwidth. If the degree of relative displacement is half the period of therepeat unit then at the interface the groove peak of one groove segmentwill be adjacent the groove trough of the other and vice versa for thetrough. The sharp break in continuity will lead to defects being pinnedalong the exposed edges in a similar fashion as for necks and gaps. Ifthe liquid crystal material is in the Defect state on both sides of theinterface the break will still lead to annihilation points for thedefect loops on each side of the interface. However the two separateannihilation points are not appreciably spaced from one another in thegroove direction. Therefore there will be minimal chance for anymaterial to form into the continuous state between the two annihilationpoints and thus the optical properties may be improved as compared togaps or necks. In actuality the slip itself may have some degree ofwidth in the groove direction so there may be a very small area ofunavoidable continuous state, but this can be minimal.

Another displacement could be, say, quarter of the repeat period. Againtaking the simple case of repeating peak and trough of roughly equalwidth the peak of one groove segment overlaps with the peak of theother. The same is true for the troughs. With this arrangement in thecase of Defect state on both sides of the interface the defect lines maynot actually come together and annihilate. Instead the defect linesmight follow the edges of the overlap smoothly into the area on theother side of the interface. This then would mean that no Continuousstate was present when not required. However the edges of the overlapwould still act as pinning sites for the defect loop when the other sideof the interface was latched into Continuous state.

As grow-back of an undesired state will occur in a pixel or sub-pixelalong the groove direction it is preferable that the majority of grooveswithin a pixel or sub pixel are arranged to have at least one breaktherein. In other words it is preferred that the device is arranged suchthat a minority of the grating within any pixel or sub-pixel comprisesgrooves not having a break within the area of the pixel or sub-pixel.Preferably less than 25%, more preferably less than 10% of the area ofthe pixel or sub-pixel comprises grooves not having a break therein.Indeed it may be beneficial to ensure that there are no areas within apixel or sub-pixel where a groove runs from one side of the pixel to theother without a break.

It may be convenient to ensure that at any given point within the pixelor sub-pixel more than one break is encountered in a groove direction.In other words as one traverses a pixel or sub pixel along the groovedirection at any point one encounters at least two breaks separatingvarious groove segments. Theoretically grow-back could occur from bothsides of a pixel or sub-pixel and with only one break the wrong statecould therefore be achieved on both sides of the break and hence thewhole of the latchable area of the pixel or sub-pixel. Having more thanone break prevents this from happening. Numerous breaks are especiallyuseful where the device is to be used in a partial latching mode.Partial latching occurs when a voltage is applied to the device, whichis insufficient to cause fill latching of a sub-pixel, but insteadcreates, within that sub-pixel, domains of liquid crystal material inone state with the rest of the material being in another state orstates. Partial latching is useful for achieving greyscale as is wellknown. Having numerous breaks in the sub-pixel along the groovedirection will aid the regular formation of partial latching domains byacting as nucleation sites as discussed above. However once the domainsare formed the numerous breaks will prevent unwanted growth or shrinkageof the domains.

For some embodiments the breaks along the groove direction will beseparated by at least double the groove pitch, i.e. each groove segmentwill have a length equal to at least double the groove pitch. Asmentioned the liquid crystal material in the vicinity of the breaks willexperience additional elastic forces compared to material located awayfrom the breaks. If the groove segments are too short these end forcesmay affect the bulk states adopted by the liquid crystal material givingincorrect alignment. A separation of breaks in the groove direction ofmore than 3 μm could be useful.

Conveniently the multistable surface alignment grating is a zenithalbistable surface alignment grating such as described in U.S. Pat. No.6,249,332 or WO02/08825. Alternatively, as described the surface couldprovide monostable alignment to the liquid crystal in which a grating isused to create a Defect state for advantageous properties other thanmultistability.

For a monostable device it is important to minimise the radius ofcurvature of the edges that form the grating grooves, since this makesthe desired state more stable. However, in practice, the radius ofcurvature is limited by manufacturing process, and may be limited to 0.1μm or greater. In such instances, it is important to introduce breaksinto the surface to prevent unwanted formation of a Continuous state, ora Defect state with the incorrect director orientation. For amultistable liquid crystal device, it is important that the radius ofcurvature of the edges is within the range suitable for formation ofeither Defect or Continuous states.

In general the invention relates to an improved surface alignmentgrating for inducing an adjacent liquid crystal material to form atleast one stable state in which defects are formed close to the surfacegrating. Therefore in another aspect of the invention there is provideda surface alignment grating for a liquid crystal material comprising agrating having a single groove direction characterised in that thegrating has a plurality of breaks along the groove direction.

In a preferred embodiment the invention relates to a multistable liquidcrystal device comprising a layer of liquid crystal material disposedbetween two cell walls, at least one region on the internal surface ofat least one cell wall comprising a multistable surface alignmentgrating having a single groove direction characterised in that thesurface alignment grating comprises a plurality of breaks along thegroove direction and in that the grating profiles on either side of thebreak in the groove direction are substantially the same.

The invention will now be described by way of example only withreference to the following drawings of which;

FIGS. 1 a and 2 b show two stable configurations for liquid crystalmaterial at a convex surface having homeotropic alignment,

FIGS. 2 a and 2 b show two stable configurations for liquid crystalmaterial at a concave surface having homeotropic alignment,

FIG. 3 shows a surface relief structure giving rise to two stablestates, a) a Continuous state and b) and Defect state,

FIGS. 4 a-4 c show plan views of three types of surface alignmentgratings used in the prior art,

FIG. 5 shows a schematic of an energy diagram for the Defect andContinuous states for a surface such as shown in FIG. 3,

FIG. 6 is a schematic of a prior art zenithal bistable device andillustrates latching between stable states,

FIG. 7 illustrates the defect lines present in a prior art grating in atransition from a Defect state (front) to a Continuous state (rear),

FIG. 8 a shows a grating having negative breaks,

FIG. 8 b shows a grating having positive breaks,

FIG. 8 c shows a grating having a slip,

FIG. 8 d shows a grating having a slip of half the grating period,

FIG. 8 e shows a grating with a repeating slip pattern,

FIG. 8 f shows a grating having two slips in close proximity,

FIG. 8 g shows a grating having a positive break combined with a slip,

FIG. 8 h shows a grating having a partial slip,

FIG. 8 i shows a grating having a change to mark-to-space ratio across adislocation line,

FIG. 8 j shows a grating having a non parallel slip,

FIG. 8 k shows a grating having an angles slip plane,

FIG. 9 shows a schematic of part of a grating shown in FIG. 8 b, thebreak comprising a gap in the groove peak, and illustrates the defectlines present in a transition from the Defect state (front) to theContinuous state (rear),

FIG. 10 shows a schematic of part of a grating shown in FIG. 8 a, thebreak comprising a neck in the groove trough, and illustrates the defectlines present in a transition from the Defect state (front) to theContinuous state (rear),

FIG. 11 shows an SEM photomicrograph of a grating with necks such asillustrated in FIGS. 8 a and 10,

FIG. 12 shows a schematic of a grating having a tapered gap in thegroove peak and illustrates the defect lines in a transition region fromthe Defect state (front) to the Continuous state (rear),

FIGS. 13 a and 13 b show a magnified photograph of a grating havingnecks, such as illustrated in FIGS. 8 a, 10 and 11, FIG. 13 a shows thepixel immediately latching to a partially latched state and FIG. 13 bshows the same pixel 60 seconds later,

FIGS. 14 a-14 c show magnified photographs of the interface between twopixels, the top pixel having gaps in the grating and the bottom pixelhaving no breaks, at various times after addressing; a) immediatelyafter latching, b) two seconds later, c) ten seconds after latching,

FIGS. 15 a and 15 b show the effect on a) latch threshold and b) thepartial latch width with increasing gap size,

FIGS. 16 a and 16 b show the effect on a) latch threshold and b) thepartial latch width with increasing distance between gaps,

FIG. 17 shows a schematic representation of a slip break such as shownin FIG. 8 d and illustrates the defects lines in a transition fromDefect state (front) to Continuous state (rear),

FIG. 18 a shows the defects following the vertical edges of the slip,

FIG. 18 b shows the defects following the top and bottom surfaces,

FIGS. 19 a-19 f show a series of photographs at different times of atest pixel with breaks (left) compared to a control pixel with no breaks(right),

FIG. 20 shows a photograph of a device made up of a repeated pattern ofthree areas of different pitch, the interface between the areas ofdifferent pitch acting as slips,

FIG. 21 shows the design of a mask suitable for creating a liquidcrystal surface alignment profile having three sub-pixels of differentpitch, each sub-pixel having a plurality of breaks in the form of slips,

FIGS. 22 a-22 c show further examples of surface alignment gratingsaccording to the present invention,

FIGS. 23 a and 23 b show SEM images of negative slips, i.e. slipscombined with gaps,

FIG. 24 shows an SEM image of a grating with positive slips,

FIG. 25 shows an arrangement wherein slips are arranged to be close topixel edges,

FIG. 26 illustrates the interpixel gap and the potential error caused bygrowback from the inter-pixel gap,

FIG. 27 shows examples of the appearance of adjacent pixels and theinter-pixel gap in various arrangements,

FIGS. 28 a and 28 b show an optical photomicrograph of a display havingslips according to the present invention a) partially latched and b)fully latched,

FIG. 29 shows the visual effects of 4 μm spaced slips as a function ofslip separation and percentage of chrome in the mask, i.e. the mark tospace ratio of the mask,

FIGS. 30 a-30 show various mask designs for producing slips having amark to space ratio is less than 50%; a) a slip with no interleave and aphase shift of 180°, b) a phase shift of less than 180° and c) anegative interleave combined with a 186° phase shift,

FIGS. 31 a and 31 b show SEM photomicrographs of two grating structuresaccording to the present invention formed using the same mask,

FIGS. 32 a and 32 b show different grating designs for use in a bistablegrating aligned device with greyscale, and

FIG. 33 shows a grating design for use with three areas of differentthreshold and the possible pixel patterns that could result.

Defect stabilised multistable liquid crystal devices are known. US Pat.No. 6,249,332 describes such a device where the surface profile of atleast one of the cell walls leads to zenithal bistability. A repeatingsurface profile is used which has a varying contour in one directiondefining concave and convex edges.

FIGS. 1 a and 1 b show side elevations for two possible directorconfigurations of liquid crystal material at a convex surface with aninternal angle of about 120° and a local homeotropic normal boundarycondition of the liquid crystal director. The short continuous linesrepresent the local orientation of the director. A dotted line is shownnormal to the director, which is included as a guide to the eye, but isalso representative of the director configuration where the surface hasa planar condition. In FIG. 1 a, a +½ strength defect (or disclination)occurs at the apex of the surface, whereas the director is continuous inthe FIG. 1 b. FIGS. 2 a and 2 b show the similar situation for a concavehomeotropic surface with an external angle of 120°. Again, there are twopossible configurations: Defect (D, FIG. 2 a) or Continuous (C, FIG. 2b). In this case the defect has strength −½ if the director is orientednormal to the surface, i.e. homeotropic alignment. The polarities of thedefect are reversed if the surface has a planar condition, with the +½for the concave surface and −½ for the convex.

The energy of the each state is related to the elastic energy of thedistortion, which in turn is related to the curvature and the localanchoring energy of the surface in addition to the elastic properties ofthe liquid crystal materials. There is an additional contribution to theenergy of the Defect states associated with the change in orderparameter of the liquid crystal material at the defect core. If there isno curvature of the surface (with uniform anchoring), the lowest energyconfiguration will be a uniform state free from defects. Without pinningsites on the surface, there is effectively an attractive force betweenoppositely charged defects will cause them to move towards each otherand annihilate. In practice, every surface has a degree of roughness,which might provide some random pinning of the defects to prevent thisannihilation. However, typical surfaces used to align liquid crystals indevices (including spin-coated polymers and photo-polymers used forgrating alignment) are relatively smooth and such pinning may be weak.The occurrence of disclinations on cooling into a nematic phase from theisotropic liquid will usually disappear a few degrees from thetransition when the sample is contained by flat surfaces with suchcoatings.

FIG. 3 shows an example of a surface profile used in U.S. Pat. No.6,249,332. The surface profile comprises a series of grooves formed by arepeating surface profile. FIG. 3 shows the cross section of the profilein the plane orthogonal to the groove direction. The combination of atleast one concave surface (the groove trough) 2 and at least one convexgrating surface (the groove ridge or peak) 4 then gives two or morebistable states in which the liquid crystal orientation is different anduniform. In the example shown in FIG. 3, the ±½ defects stabilise onetilt of the director, and the Continuous state stabilises a second tilt.In both cases, the tilt is uniform in a plane parallel to the surface(represented by the dotted line S-S′ in FIG. 3) in close vicinity to theplane of the grating (usually within a distance less than or equal tothe grating pitch). For a homeotropic boundary condition the Defectstate has a low tilt and the Continuous state has a high tilt, oftenclose to 90°. If the surface has a planar condition (in the planeperpendicular to the grating), the Defect state has high tilt and thecontinuous state low tilt.

Other grating designs are described in WO02/08825, wherein the gratinghas three or more defect sites, such as two convex and one concaveedges, two concave and one convex or two concave and two convex. In suchinstances, multiple states can exist corresponding to the defect pairshaving different relative positions, leading to different opticalconfigurations and greyscale.

In each of these cases the grating is continuous in a direction normalto the zenithal plane containing the director. A representative planview of a typical grating used in the prior art is shown in FIG. 4 awhere the shaded area represents groove troughs and the light areasrepresent groove peaks. Typical pitch p is in the range 0.1 μm to 5 μm,preferably in the range 0.4 μm to 1.4 μm, or 0.6 μm to 1.1 μm. Thegroove direction (represented by the unit vector g) may be constantwithin each pixel of the device, or may vary within the pixel. However,the groove direction g will be constant over length scales of below 15microns for the majority of a pixel. The grating can be furthercharacterised using the mark to space ratio (c/p) and degree ofasymmetry, which in turn influences the curvature of the leading andtrailing edges of the surface profile. In the Defect state, the ±½defects are formed as lines (ignoring the thickness of the defect coreas a realistic representation) parallel to the groove direction g. Theliquid crystal director has a component perpendicular to g but nocomponent parallel to g. In the Continuous state, the director will havea different component perpendicular to g, but again will have nocomponent parallel to g.

Included in the device described in WO01/40853 are changes of the groovedirection g over length scales of less than 15 microns. These aredesigned to vary the director orientations within each pixel, creatingmicro-domains or pixel areas that can, for example, give scattering ofincident light. Examples are the bi-directional grid shown in FIG. 4 band the bi-grating of FIG. 4 c. Both examples give a locally varyingdirection to the orientation of the director in the Defect state in theplane parallel to the surfaces. The aim is to lead to a structure thathas components of the in-plane director component of the low tilt statein mutually perpendicular directions. For example, this may then lead toan optical configuration that causes substantially more scattering whenin the Defect state than in the Continuous state.

The energy levels for the two states are represented schematically inFIG. 5. The energy barrier 6 between the states is related to the energyassociated with annihilating and creating defects. The surface may bereadily designed to give either symmetric (dotted line) or asymmetric(continuous line) latching characteristics. Both cases are bistable,where the energy barriers U_(D) and U_(C) are sufficiently high, even ifthey are not symmetrical. If the barriers are low, then externaldisturbances may cause unwanted transition from one state to the other.For example, with a positive dielectric anisotropy material, the appliedfields used whilst multiplexing the other lines of a device will tend tostabilise the Continuous state. If U_(C) is low, grow-back of theContinuous state will then occur. Alternatively, at high temperatures,the liquid crystal order parameter S is low so the energy of the Defectstate becomes lower. If the surface is designed to give Continuous statestability at ambient temperatures, then at some elevated temperature theenergies become symmetric and eventually give a mono-stable Defect stateclose to the clearing point. Reverse ionic fields, built up by themigration of free ions during addressing, will also play a part.

An example of a practical device configuration is shown in FIG. 6. Themultistable surface relief structure 10 is used opposite a flatmonostable homeotropic surface 12 to give either a vertically aligned ora hybrid aligned nematic. When the sample is placed between crossedpolarisers, arranged at an angle to the grating, the change in opticalretardation leads to a difference in transmission or reflection.Latching between the states may be done using electrical pulses ofappropriate polarity, 14. It is customary to use bipolar pulses tomaintain DC balance, wherein it is the polarity of the trailing pulsethat determines the state.

There are different mechanisms for latching between the states. Theelastic distortion in the Defect and Continuous states results in aflexo-electric polarisation for polar liquid crystals with certainmolecular shape anisotropies. Moreover, the core of the defectrepresents a locally “melted” nematic liquid crystal, where the orderparameter is effectively zero. The gradient of order parameter in thevicinity of the defect core also leads to a polarisation (the so calledordo-electricity). The symmetry of the ±½ defects means that the defecthas a net polarisation, resulting either from the localflexoelectricity, the ordo-electricity or their combined effect. Othermechanisms for a polar response may be provided, including the effect ofbreaking n to −n symmetry at the surface, and the effect of ionicimpurities. The cumulative effect is a surface polarisation P_(s) at thegrating surface which couples to the applied DC field to inducelatching. It should be noted that the device also has a bulkflexo-electricity in one of the states (the Hybrid aligned state), butthe effect of this is very weak, and in practice may be screened by themobile ionic impurities in the bulk of the liquid crystal.

FIG. 7 is a 3D schematic of the grating surface at the boundary betweenthe Defect state, towards the front of the figure as shown, andContinuous state towards the rear. Dotted lines represent convex surfacecurvature and dashed lines represent concave. In the defect D state(front), +½ disclinations 20 occur close to a convex surface, and −½disclinations 22 occur close to a concave surface. The disclinationsoccur along the surface giving rise to defect lines. Where an area ofDefect state is adjacent an area of Continuous state however there mustbe a point where the Defects are removed. “A” represents the point ofannihilation for the two disclinations, behind which the state iscontinuous C. Without any deliberate form of pinning, the annihilationpoint may traverse freely across the surface 26 to reduce the totalenergy of the system. If the surface is without any pinning this maylead to unwanted formation of one state instead of the other (D in C orC in D) as the Defect lines (or Defect loop as it may be termed) shrinksor grows. If the two states have equal energy then the desired statewill be maintained. However, if some external influence acts to disturbthe pixel, whether it is mechanically induced flow, or a transientelectric field associated with the multiplex addressing signals appliedto the device or a reverse ionic induced field, then the undesired statemay result. Moreover, the properties of the system that determine therelative energies of the alignment states may vary, for example due tochanges of the ambient temperature. The elastic constants, orderparameter of the liquid crystal and the surface anchoring energydecrease with increasing temperature, and can cause the relativeenergies of the bistable states to change. Thus, a surface designed togive appropriate energies for the D and C states under normal operatingtemperatures may become monostable at elevated temperatures (usually tothe D state). In practice, of course the surface used to form thegrating will have microscopic roughness that have sufficient pinningstrength that they prevent grow-back of the unwanted state in practicaldevices. Note, the grating shape used in this example is trapezoidal:each part of the grating repeat pattern has two convex and two concaveedges. The defect might occur either at the obtuse convex and concaveedges as shown in the figure, or on the more acute edges of the gratingsurface (i.e at the top and bottom of the vertical edge rather than thesloping edge).

The basis of the invention is to create dislocations or breaks to thesurface treatment used to align liquid crystals in order to create siteswhich pin the defect lines in the groove direction. These breaks may beregularly, randomly or pseudo-randomly spaced across the grating.Examples of such breaks are shown in FIG. 8, although this set ofexamples is not exclusive. In each case, the grating has substantially asingle groove direction and any changes to the alignment of the directorin either state are kept relatively small. Preferably, each break shouldinclude a concave or convex edge to the surface profile that has acomponent perpendicular to the plane of the device. That is, the breakintroduces an edge that runs at least partially from the concave andconvex edges of the surface profile of the repeat unit that forms thealignment grating. The breaks effectively introduce corners into thesurface profile.

FIG. 8 a shows a grating having what may be termed negative breaks orgaps. The grating is a series of grooves each having a ridge, unshadedareas 30 and a trough, shaded areas 32. At certain points the ridgeshave a gap 34 therein. The gap then forms a break or discontinuity inthe profile of the ridge in the groove direction. Defects will then bepinned at the edges of the gaps as will be described. As can be seen thegaps do not have to be aligned so that a gap in one ridge is adjacentanother gap.

FIG. 8 b shows a similar type of break but where the ridges are linkedby necks 36 in the troughs. Again, this positive break introduces defectpinning sites at the interface of the necks and the troughs.

FIG. 8 c shows an example of a slip in the grating. This can be thoughtof as a displacement of the top part of the grating relative to thebottom part or a dislocation of the grating phase. In this instancethere is a small displacement, s, which means that the exposed edges 38of the ridges will act to pin the defect lines present in the Defectstate. FIG. 8 d shows a larger slip where the peak or ridge of thegrating meets the trough of the displaced part of the grating. Note theterms slip and displacement are used only to give an indication of thetype of break. No actual movement of the grating is necessarily implied,as the skilled person is aware there are various methods for forming analignment grating such as using photoresist materials and a grating withthis pattern could be formed using a suitable mask. Note, thedislocation may occur suddenly, or over a small distance parallel to thegroove direction. The curvature induced by the dislocation is in theplane of the cell creates an edge that is perpendicular to the groovedirection and has a component that is parallel to the cell normal. Thiscurvature should be sufficient to pin the defect state and annihilationpoints in the vicinity of the slip. In practice, the curvature isgreater than 1π/μm, preferably greater than 2π/μm and often greater than5π/μm or in other words the radius of curvature is less than 2 μm,preferably less than 1 μm or even less than 0.4 μm.

FIG. 8 e shows a repeating slip pattern where a unit consisting of onegrating displaced with respect to another is repeated. FIGS. 8 f and 8 gshow gratings with a mixture of neck like and gap like breaks.

FIG. 8 f shows two slip dislocations in near proximity that doubles thenumber of pinning sites, thereby helping to ensure Defect state pinningoccurs even if one slip on its own is not effective.

FIG. 8 g shows a mixed dislocation involving both slips and necks. Thistype of break may help ensure that the corners remain well defined inpractice. Some grating production techniques may not be able toreplicate the rapid change in grating shape at the slip, tending insteadto give a more gentle, “S”-like slip that has no or weak Defect statepinning properties. This arrangement would help ensure that the minimumdegree of curvature at the slip would be sufficient to pin the Defectstate.

FIG. 8 h shows a partial slip, bordered by areas with a change inmark-to-space ratio of the grating. This type of slip may be usefulwhere regions of multistable operation are bounded by monostableregions. It may be preferable to have no breaks in the monostable regionif, for example, the Continuous state is required in that region.

FIG. 8 i shows a break in which there is a change in mark-to-space ratioof the grooves across a dislocation line. This may lead to differentDefect states either side of the break.

FIG. 8 j shows a slip dislocation that is not parallel to the normal ofthe groove direction. In this example, the angle of the slips is alsoshown to change, forming a zig-zag pattern across the region. As withFIG. 8 c, the dislocation in this instance has no finite width. Inpractice, it may be difficult to reproduce the fine features in thegrooves required to ensure this. Therefore, the situation shown in FIG.8 k may happen in practical embodiments. This shows an angled slip planein which the rectangular ends to each ridge (or groove) are maintained.This will have a similar effect to combining the effect of gaps, necksand slips together.

A gap is shown in greater detail in FIG. 9. This is a 3D representationof the grating surface that includes a gap and the shows position of thedefects in the Defect state close to the grating surface. Using the samenumerals as FIG. 7 the +½ defect 20 is shown using horizontal shadingand the −½ defect 22 with vertical. Note that the diagram shows thesituation with homeotropic boundary conditions, the position of thedefects would be reversed with a locally planar alignment. The gap 40creates a convex surface that joins the horizontal top convex edge ofthe groove to the horizontal concave edge at the bottom of the groove.The lowest energy configuration for the +½ defect 20 is to follow thisedge, bending around the top corner at B, and annihilating at the bottomcorner of the break at the annihilation point A where it meets the −½defect 22. As described earlier, the director configuration close to thegrating in the Defect state has a low surface pretilt. FIG. 9 shows thesituation where there are no defects behind the gap; i.e. the gap marksthe boundary between the Defect state (front) and Continuous state(rear), of low surface tilt and high surface tilt respectively. Unlikethe situation shown in FIG. 7 the annihilation point is no longer freeto move over the surface. This provides a pinning site for the defectlines in the Defect state, since there is an energy cost associated withmoving the defect away from the vertical edge.

The asymmetry of the trapezoidal grating in FIG. 9 is sufficient toalways cause the correct sign of bend at B. Even if a symmetricalgrating (e.g. with a rectangular cross section) is used theconfiguration shown is still the lowest energy since the overall defectlength is minimised. Moreover, the array of defects induced by thegrating helps ensure that defects are formed in the same pattern at eachbreak.

FIG. 10 shows the opposite break, a positive “neck” in the grating. Theeffect of this is similar to that of the gap, except now the verticaledge introduced by the break according to the present invention isconcave, and the annihilation point occurs at the top corner of thebreak.

A grating with this structure was produced on top of an ITO coated glasssubstrate in the following manner. A chrome mask designed with thestructure shown in FIG. 8 b was produced with alternating strips of 500nm chrome, 500 nm clear, and with gaps 500 nm wide spaced 12 μm apart(and shifted through 4 μm on adjacent grooves). A positive photo-resistmaterial was spun coated onto a second substrate, baked, the mask put incontact with the photo-resist and exposed to deep UV light at a slightoff normal angle (8°) in the direction perpendicular to the groovedirection g. After developing the substrate and etching the exposedphoto-resist away, the grating master forms a negative of the desiredstructure. This master was then used to emboss the desired structure onthe ITO coated substrate, either directly or using an appropriate numberof copies made from the master. An example of an embossed grating isshown in the SEM in FIG. 11. With embossing, the troughs of the gratingare formed from the ridges in the master or even generation copiesthereof; hence a master that includes gaps will lead to a final gratingstructure with necks.

This figure shows typical surface curvatures that can give bistability.The pitch of this sample was 1 μm. The curvature of the neck structuresadded into the grating according to the present invention is more than|1 π/μm|, being approximately |2 π/μm| in the example shown. In somesituations, e.g. for very high anchoring energy, or low order parameterS, it may be advantageous to use a lower curvature, but less than0.2π/μm is likely to be too weak a curvature to cause any substantialpinning energy for the defects. Curvatures higher than |2 π/μm| (or aradius of curvature of less than 1 μm) are also suitable with an upperlimit (dictated by the liquid crystal elasticity and local surfaceanchoring energy) that is not found in practice using the gratingfabrication method described. For a multi-stable device, the relativeenergies of the different states will be dictated by the relationshipbetween the magnitude of the surface curvature, pitch and amplitude ofthe grating, local surface anchoring energies and the elastic propertiesof the liquid crystal material.

The breaks may have different strengths, as indicated in FIG. 12. Inthis example, the degree of bend occurring at B is related to thegradient of the convex edge of the grating from B to A as shown.Moreover, the break may be incomplete, in which the concave/convex edgedoes not meet at a point.

A cell was constructed using the grating substrate of FIG. 11 spaced at5 μm from a flat, ITO coated glass substrate with a mono-stablehomeotropic treatment. After filling with the commercial liquid crystalmaterial MLC 6204_(—)000, the cell was observed in transmission betweencrossed polarisers on an optical microscope with the polarisersapproximately at 45° to the grating direction. Latching between theDefect (white) and Continuous (Black) state occurred following bi-polarelectrical pulses applied to the ITO electrodes that were of appropriateamplitude, duration and polarity. It was noted that latching was veryuniform compared to a control area where there was no breaks in thegrating. The pulse amplitude was then lowered so as to cause partiallatching from one state to the other. FIG. 13 a shows the sampleimmediately after a pulse that latched approximately 40% of the pixelarea to the Continuous (black) state. The grating breaks are clearlyvisible in this diagram as an array of points, since they form a small(approximately 1 μm²) domain of Continuous state at each point. Aphotograph of the same area was taken a minute later, as shown in FIG.13 b. Although there is some shrinkage of the defect domains, it is towithin a satisfactory level. Areas without breaks grew almostimmediately to the Continuous state, as illustrated with respect toFIGS. 14 a-14 c.

A similar sample, but this time arranged to give gaps in the grating,was heated to 50° C., 15° C. below the clearing point T_(NI). At thiselevated temperature the sample is inherently more Defect D statestable. The pixel having breaks 50 was partially latched into the Defectstate and compared with a control area 52 with no such breaks. FIG. 14 ashows the situation after latching. It can be seen observed that thesame voltage applied to both the grating area 50 having breaks and thecontrol area 52 caused more latching to the Defect state in the areawith breaks acting as defect loop pinning sites. That is, the thresholdfor C to D latching is reduced by the presence of the gaps, since thebreak acts as a nucleation site for the transition. This has theadvantage of allowing lower voltage operation or faster line addresstimes as compared to conventional devices. Two seconds after the pulse,as shown in FIG. 14 b, the area without gaps begins to show significantgrow-back to the more stable Defect state, i.e. white lines can be seenbeginning to encroach into area 52, whereas there is little change inthe area 50 with gaps. Within 10 seconds, as shown in FIG. 14 c, theunwanted Defect state had spread wholly across the control area 52 ofthe pixel, whereas the change to the broken grating area 50 was minimal.At these elevated temperatures, the D state grow-back, even for pixelswholly latched into the Continuous state, can be severely limiting tothe device. The temperature range of satisfactory operation haspreviously been found to be 45° C. at most. Use of the broken gratings,i.e. gratings having breaks in the grooves, has allowed devices to bedriven to within 3° C. of the clearing point (which is over 60° C. forMLC 6204_(—)000).

FIGS. 15 a and 15 b and 16 a and 16 b illustrate the positive effect ofbreaks on device latching thresholds, both as a function of gap size anddensity, i.e. distance between breaks. The control sample without gapsis plotted as a gap 0 μm. There is a slight tendency for the latchingvoltages to decrease with increasing size of gap. This is possibly dueto the increased effective field in the vicinity of the gap, as there isless dielectric drop across the grating, which then acts as a principlesite for nucleation of latching.

Referring back to the high magnification photograph of FIGS. 14 a-14 cit can be seen that the gap or neck can lead to a small area ofContinuous state centered on the break. Although very small, this mightcause a significant change in the optical properties of the Defect Dstate, potentially leading to reduced transmissivity, or reflectivity,of the display device. Moreover, the gap or neck acts as a nucleationsite for growth of the Continuous state. This might give unwantedgrow-back to C, induced, for example by the RMS effect of the appliedfield applied to other lines during multiplexing. It is a further aim ofthis invention, therefore, to provide breaks which act to pin the defectlines or defect loops in the groove direction, but without the unwantedeffect of providing mono-stable areas. This may be achieved using thetype of break structure referred to as a slip, as shown in FIGS. 8 c and8 d.

A slip grating break is shown schematically in FIG. 17. Again theboundary between Defect (front) and Continuous (rear) states is shown,with only the ±½ defects shown, 20 & 22, rather than the directorconfiguration. Here, the ridge of the groove is shifted suddenly,creating a phase difference in the periodic structure, but maintainingproperties such as the pitch, mark to space ratio and grating shape.This slip then introduces a concave edge and a convex edge together, butshifted through the distance, s. Either edge may act to pin the defects,depending on whether the front area is Defect, the rear Continuous (asshown), or the front area is Continuous and the rear is Defect.

FIGS. 18 a and 18 b show two possible configurations for theconfiguration at a slip where the pixel area is in the Defect state onboth sides of the slip. In the case shown in FIG. 18 a, the defectsfollow the vertical edges of the slip and create two annihilationpoints. This configuration may lead to an unwanted area of Continuousstate—particularly as the slip is shown to have a depth in the directionparallel to the groove direction g in this example, which might occur inpractical manufacture. However, the ±½ defects 20, 22 can alternativelykeep to the top convex and bottom concave surfaces, and merely followthe change in phase induced by the grating shape. This configuration ismost likely if the distance s is less than the groove amplitude(provided the energy cost of the defect bend B is sufficiently low). Fora full grating (rather than the single ridges shown here) the maximumphase shift is the greater of c/p or (1-c/p), which is usually muchlower than the grating amplitude. The optimum phase of slip for agrating will depend upon the asymmetry and curvature of the surfaceprofile repeat unit.

Hence, slips have all of the advantages found with gaps or necks butmaintain the good optical performance of the device by avoiding orminimising the possibility of areas of incorrect state being formed.That is, the slips cause the least disruption to the directorconfiguration in the two states, which remains substantially uniformlyaligned at the grating surface. This is shown in FIGS. 19 a-19 f, wherea grid of gratings with slips is shown on the left (the groove directionbeing vertical as shown), next to a control area without slips. FIG. 19a shows the situation immediately after latching with FIGS. 19 b, c, d,e and f showing images after 1, 2, 3, 4 and 5 seconds respectively. Itcan be seen from FIG. 19 a that the area having breaks achieves partiallatching whilst the control area does not, illustrating that thepresence of breaks aids in formation and maintenance of domains. It canbe clearly seen, looking at the sequence of images, that the whole pixelwith breaks was able to maintain a partial latched state withoutsubstantial grow-back, even at this elevated temperature, whilst thecontrol area showed considerable grow-back of the lighter Defect state.FIG. 20 shows a panel comprising a repeat of three areas of differentgrating pitch. The groove direction g runs up and down the page. Area ahas a repeat unit having a first pitch, in this case 0.6 μm, area b adifferent pitch, 0.8 μm and area c a different pitch again 1.0 μm. Eacharea has a length equal to 12 μm. Areas a, b, and c therefore representsub-pixels having different profiles so as to have different latchingcharacteristics allowing areas of both states Continuous and Defect tobe formed. FIG. 20 shows the situation after area a has been latched toDefect state, whilst areas b and c are Continuous state. Because thethree pitches are different the interface between the three areaseffectively result in slips of varying phase shift at each edge. Howeveras the pitch varies there are areas where grooves of the adjacent areasalmost overlap. It can be seen from the image that area b had fivedistinct bands. This is due to grow-back from area a at these placeswhere the alignment is such so that there is no effective slip. Thissuggests that, for typical grating manufacturing methods, the phaseshift of the slip should be in the range π/2 to π.

FIG. 21 shows how slips might be used in a grating mask designed to givethree different pitches. With such a device, both 4 error-free greys anda high degree of error-containing greys will be achieved. Three subpixel areas are therefore formed, areas A, B and C, each having adifferent pitch, respectively 0.6 μm, 0.8 μm and 1.0 μm. The groovedirection of each sub-pixel is from side to side as shown. It canclearly be seen however that each area has a plurality of groovesegments separated by slips.

Such a device could then display three error free grey levels per pixelusing the different latching thresholds of each sub-pixel area. Howevergrow back from one sub-pixel to another or from the inter-pixel gap isminimised by the presence of breaks. Further the presence of breaks aidthe partial latching window allowing error free greys to be achieved.

FIGS. 22 a-22 c show three further examples of gratings designedaccording to the present invention. FIG. 22 a shows an example thatcombines the present invention with the type of device described inWO01/40853. In this embodiment, the device has a plurality of regionsand the grating groove direction is different in adjacent regions.Within each region, there is a slip break in which the grating groovedirection g remains the same or substantially the same either side ofthe dislocation plane. If the device were designed according toWO01/40853 such that each of the regions with a single groove directionwas 15 μm or less in dimension, then the addition of the breaksaccording to the present invention would lead to improved performance ofthe device. For example, the device may be required to cause scatteringin the Defect state, and much less scattering in the Continuous state.The slips act to stabilise the Defect state, prevent growback to theContinuous state (or vice versa) and thereby improve the operatingwindow.

FIG. 22 b is a grating designed according to the present inventionwherein slips are repeated at a distance equal to or less than thegrating pitch. If the phase of the slips were 180° the resultingstructure would be a bi-grating similar to that of FIG. 4 c. Such astructure would no longer have a single groove direction at any point,but would have two orthogonal grating grooves. This would be equivalentto an embodiment of WO01/40853. However, because the phase of the slipis significantly less than 180°, preferably being about 90°, the singlegroove direction g is retained. This means that the structure has a veryhigh density of Defect nucleation and pinning sites, but that the liquidcrystal director has a uniform orientation within the region in bothDefect and Continuous states.

FIG. 22 c shows a slip plane wherein there is a slight change in groovedirection on either side of the break. This is the type of change thatmay occur where there is a gradual change in groove direction fromregion to region. Alternatively, the grooves may be misaligned slightlydue to some experimental error.

FIGS. 23 a and 23 b show SEM images of two gratings with negative slipsoccurring every 4 μm. FIG. 23 a shows a grating having a pitch of 1 μmwhereas FIG. 23 b shows the boundary between an area of pitch 0.8 μm onthe left and 0.6 μm on the right.

FIG. 24 shows an SEM image of a grating having a positive slip for agrating with 0.6 μm pitch.

It is also possible that there can be advantage to the opticaldisruption caused by the director distortion about a break. For example,the optimum gap size is likely to be of the order of half the pitch(i.e. 0.25 to 0.75 μm). However, if larger breaks are used with a highdensity, it may be possible to use the deformation of the directoraround each break to give some scattering of the light. The scatteringwill be higher in the Defect state (which should therefore be made thewhite state) than the Continuous state. This scattering will act as aninternal diffuser. Taken to the extreme, it may be possible to designthe breaks to allow a flat reflector to be used, thereby reducing thenumber of steps in the cell process, with the expense of the diffuserappearing in the mask design only.

As mentioned a useful embodiment of the invention is the introduction ofgrating phase shifts, or slips, of between 90° and 180° phasedifference. The slips are arranged to occur at least close to the pixeledges running perpendicular to the groove direction g. If the slips arenot near the edges of the pixel the amount of pixel that may growback tothe wrong state is increased, however too high a density of breaks mayhave an effect on pixel contrast.

Imagine a device where the Defect state is arranged as a Twisted Nematic(TN) configuration and is the black state. The Continuous state is aHybrid Aligned Nematic (HAN) state and appears white. If the pixel pitchis P and the average slip spacing is x then the number of slips perpixel, N, is:N=INT(P/x)

For P>>x this simplifies to;N=P/x.   {eqn 1}

In the worse case scenarios the pixel edge that is parallel to the slip(and hence perpendicular to g) will be close to the slip but on thewrong side. In this case, a region that is almost x wide may be in thewrong state. If this situation occurs on both sides of the pixel thenthere is a total brightness error per pixel ε1=2x/P. Typically, theinterpixel gap cools into the Defect state and largely remains unchangedduring driving. After latching a pixel to the Continuous state (white)the Continuous domains may shrink back to the closest slip to the pixeledge, as the Defect state spreads from the inter-pixel gap.

To minimise the error ε1, it would be ideal to include a slip close toeach pixel edge, as shown in FIG. 25. The closer to the edge of thepixel, the more effective the slip would be. Indeed, it may beadvantageous for the slip to be located slightly outside the pixel area,since this area may still be addressable due to the in-plane electricfields that occur at the pixel edges.

Accurately aligning the slips with the pixel edges may add cost to thefabrication, requiring expensive mask alignment equipment. It may not bepractical to use this approach if the grating is embossed into aphoto-polymer layer over the electrodes. Moreover, this requires aseparate grating design for each electrode arrangement. In practice,therefore, the grating is designed to have a high slip density, to helpensure that a slip occurs close to each interpixel gap. This also helpsreduce the spread of the unwanted state when nucleated from defects inthe cell, changes in temperature or through mechanically induced flow.

The errors that may occur at an inter-pixel gap are shown schematicallyin FIG. 26. These errors will change across the display in a directionparallel to g, unless there is a precise match between the pixel pitchand the slip separation. Two possible arrangements of the same gratingpattern are shown on the left and right sides of FIG. 27. Suchdifferences may occur at different pixels on a display, for example inpixel A in FIG. 27 and pixel A+n, where n is some integer. FIG. 27 alsoshows the appearance of the inter-pixel gap for either pixel where theinterpixel gap is white, i.e. the white state is the lower energy state(shown in the top pixels), and when the black state is the lower energystate (the lower part of the figure). Clearly, the apparent width of theinter-pixel gap may change, depending on the arrangement of slips withrespect to the electrodes: The white inter-pixel gap between pixelslatched black may have a width X or Y as shown in FIG. 27.

If the distance between the slips is kept small, then the differencebetween X and Y will also be small. However, for regularly spaced slipsthis may still lead to noticeable artefacts. For example, Moiré fringesmay occur as the inter-pixel width oscillates consistently from pixel topixel. Such fringes can be noticeable even if the difference in gapvaries by a few microns or so, if the variation is regular and on alength scale that is a few mm or so. Two approaches help resolve thisproblem are to orient the grating and the slips at an angle to theinter-pixel gaps, and ensuring that the slips are spaced randomly. Azenithal bistable device display was fabricated and arranged to be TN inthe Defect state and HAN in the Continuous state, corresponding to whiteand black states respectively. The display uses a grating designed withslips pseudo-randomly spaced at 7 μm, 8 μm and 9 μm. Care was taken toensure that the same average spacing was maintained over length scalescorresponding to the pixel size at most. For example, the same number ofslips with 7 μm, 8 μm and 9 μm period were used for length scales ofabout 100 μm (eg five slips of period 7 μm, and five periods of 8 μm andfive at 9 μm but in a random sequence, such as7:7:9:8:8:9:7:8:9:8:9;7:8:9:7). This pattern may then also be changedfor the next sequence, and so on. Optical photomicrographs of thisdisplay are shown in FIGS. 28 a and 28 b, with FIG. 28 a showing apartially latched state and FIG. 28 b showing the display fully latchedinto the Defect state. It can be seen that in the partially latchedstate the slips pin the domains of Defect and Continuous states.

It can also be seen in FIG. 28 b that each slip also leads to abrightness error ε2 from its finite thickness. In the present examples,the Defect state appears white, and the slip appears as a dark band.This may be due to the formation of a thin line of Continuous state atthe slip, or due to the refractive effect associated with the change inrefractive index around the slip. In either case, the slip causes areduction in total brightness across a white pixel, or increases thetransmission of a black pixel. Clearly this error can lead to adegradation of display performance, including reduction of brightnessand/or reduction of contrast ratio. For this reason, it is important todesign a grating to be used as a bistable liquid crystal alignment layeraccording to the present invention in which ε2 is minimised.

FIG. 29 shows several practical examples of Zenithal Bistable Gratingcells using the present invention. The figure shows photomicrographs of4 μm spaced slips as a function of slip separation (which varies fromleft to right) and percentage of chrome in the mask, i.e. mark to spaceratio, which varies from top to bottom. All cells were latched to theDefect state to appear white. The slip appearance was found to depend ona number of factors, including the design of the mask and thephotolithographic process used to define the grating.

The examples shown in FIG. 29 are variation of the mask parameters, markto space (top to bottom) and interleave of the slip. A positiveinterleave is shown in FIG. 8 g: the slip is combined with a row ofgaps. If the interleave is negative, the slip will be combined with arow of necks, for example as shown in FIG. 18. Where the mark to spaceratio is less than 50%, it is possible to have a negative interleavesuch as that shown in FIG. 30 c. Note, FIGS. 8 a-8 j and 30 a-30 c showthe design for two-level photolithography masks and not the finalgrating produced by that mask in photolithography or other manufacturingprocess (e.g. ruling, embossing etc). FIG. 29 shows that the apparentwidth of the slip ε2 is lowest for a +250 nm inter-leave if the mark tospace is 50%) and may also be reduced by using a lower mark to spaceratio.

These results are very specific to the conditions used to fabricate thegrating. FIGS. 31 a and 31 b show SEM photomicrographs of two differentgratings of the present invention, produced using the same mask (0.8 μmpitch, 50% chrome and 50% clear, 180° phase change at each slip and a 0μm interleave) but embossed from masters produced using differentexposure conditions. In FIG. 31 a the slips are formed from a ridgerunning perpendicular to the grating grooves that is the same height asthe grooves. In practice is has been found that this can lead to anoticeable width of slip, W, and causes a reduction in the brightness ofcontrast of the display. In the grating shown in FIG. 31 b the slipsform a much lower ridge feature and the apparent optical width, W, ofthe slip was much lower. Therefore, for this type of shape, the slipconditions found to give the narrowest slip (ie ε2 minimised) was thatof FIG. 31 b).

However, the fabrication conditions are also set to give many otherrequired properties from the grating, and it may not be possible toremove ε2 altogether in practice. This leads to a conflict in the designbetween having a high number of slips to minimise ε1 at the pixel edges,and having a low number of slips to reduce the overall effect of ε2.Assuming for simplicity, that the slip is fully black over the distanceW (see FIG. 28), then the ε2 error for a white pixel is proportional tothe total number of slips across the pixel multiplied by the pixelwidth, thereby giving:ε2=W/x   {eqn 2}

Equating equations 1 and 2 gives the slip density x where the totalerror is minimised:

$x = \sqrt{\frac{P \cdot W}{2}}$

Hence, the grating design may be adjusted to minimise the error due toslips according to the pixel pitch that it will be used with. Inpractice, it is preferable for the design to be flexible, regardless ofpixel size and pitch. Hence, the grating will be designed to worksatisfactorily for a range of different pixel pitches, and is typicallyin the range 2 μm to 15 μm, and commonly in the range 4 μm to 10 μm fordisplays of 400 dpi to 65 dpi.

FIGS. 32 a and 32 b show how the present invention may be used inconjunction with a grating designed to have different properties,perhaps to provide for greyscale. In this instance, a given area mayhave an adjacent area in the opposite, more favoured state. At theinterface between the two areas, the phase of the grating may vary from0° to 180°. Where the phase change is insufficient to cause Defect statepinning (eg it is less than 90°), the Continuous state may spread intothe first area, thereby leading to an undesired transmission state.Hence, a slip may be located close to this interface to help ensure thatthe lower energy state does not spread into the area after latching intothe higher energy state.

The slips may be put close to each side of each separate area as shownin FIG. 32 a. For a grating with different areas designed to givegreyscale, there may be a particular sequence of latching the differentareas that would be known prior to the grating design. For example, (seeFIG. 33) grating design 1 may latch into the higher energy, dark stateat a lower electrical pulse energy (τV) than grating shape 2, which inturn latches dark before grating shape 3. Knowing this sequence, it ispossible to protect all of the possible states using only 1 slip ingrating shape 2, and no slips in grating shape 3.

In summary then the advantages of the invention include;

Wider temperature range—bistable operation to within 5° C. of theclearing point T_(NI) has been observed for the first time to theinventor's knowledge. As the order parameter decreases at elevatedtemperatures the Defect state becomes more stable, so D state grow backmight occur. This is hindered by the “vertical” edge of the break.

Reduction of sensitivity to RMS induced latching or grow-back. Thishelps maintain a high aperture ratio for the pixels (i.e. brightness andcontrast) in a multiplexed display despite application of a field. Inpractice this allows higher data voltages to be used and/or more linesto be addressed, and more regularly.

Widening of the bistability window—this enables bistability to bemaintained for a wider range of grating shapes. This can be used to givelower voltage latching to one of the states. Although blanking to theother state may then require a longer time to latch, low voltageoperation of the device with a fast overall frame time can be obtained.

The density of nucleation sites may also be controlled to give widerpartial switch regions in a controllable fashion (rather than relying onvariations across the cell) or reduced partial latching voltages. Thisin turn allows either more analogue levels to be achieved, or lower datavoltages to be used (and hence reduced power).

1. A liquid crystal device comprising: two cell walls; and a layer ofliquid crystal material disposed between said two cell walls, at leastone region on an internal surface of at least one cell wall comprising asurface alignment grating, said grating comprised of a plurality ofgrooves extending parallel to only a substantially single groovedirection, wherein each of the surface alignment grating groovesincludes at least one break extending along the groove, each of saidbreaks comprised of at least one discontinuity in each of said grooves,wherein the single groove direction is substantially the same on bothsides of each of said plurality of breaks.
 2. A liquid crystal device asclaimed in claim l wherein the surface alignment profile on one side ofthe break is substantially the same as the surface alignment profile onthe other side of the break along the groove direction.
 3. A liquidcrystal device as claimed in claim 1 wherein the break has a width inthe groove direction that is less than 5 μm.
 4. A liquid crystal deviceas claimed in claim 3 wherein the break has a width in the groovedirection that is less than 0.25 μm.
 5. A liquid crystal device asclaimed in claim 1 wherein the device is a multistable device and thesurface alignment grating comprises a multistable surface alignmentgrating.
 6. A liquid crystal device as claimed in claim 1 wherein thebreaks comprise a surface feature having a profile in a plane containingthe grating normal and the groove direction which has at least oneconcave and at least one convex edge.
 7. A liquid crystal device asclaimed in claim 6 wherein the radius of curvature of the at least oneconvex or at least one concave edge is less than 2 μm.
 8. A liquidcrystal device as claimed in claim 6 wherein the at least one convex andat least one concave edge are joined by a surface, at least part ofwhich is substantially parallel to the grating normal.
 9. A liquidcrystal device as claimed in claim 1 wherein at least one breakcomprises an area where the profile of a groove trough is raised.
 10. Aliquid crystal device as claimed in claim 9 wherein the profile of thegroove trough at a break is raised to the same level as the groove peak.11. A liquid crystal device as claimed in claim 1 wherein at least onebreak comprises an area where the profile of a groove peak is lowered.12. A liquid crystal device as claimed in claim 11 wherein the profileof the groove peak at the break is level with the groove trough.
 13. Aliquid crystal device as claimed in claim 1 wherein at least one breakcomprises an area where the grooves on each side of the break aredisplaced relative to each other in a direction having a componentperpendicular to the groove direction.
 14. A liquid crystal device asclaimed in claim 13 wherein the grooves on either side of the break ofdisplaced relative to each other in a direction substantiallyperpendicular to the groove direction.
 15. A liquid crystal device asclaimed in claim 13 wherein the relative displacement is substantiallyequal to half the groove period.
 16. A liquid crystal device as claimedin claim 1 wherein the device is arranged such that within any pixel orsub-pixel less than 25% of the grating within that area comprisesgrooves not having a break within the area of the pixel or sub-pixel.17. A liquid crystal device as claimed in claim 16 wherein the device isarranged such that within any pixel or sub-pixel less than 10% of thegrating within that area comprises grooves not having a break within thearea of the pixel or sub-pixel.
 18. A liquid crystal device as claimedin claim 1 wherein the device is arranged such that at any point withinthe pixel or sub-pixel more than one break is encountered along thegroove direction.
 19. A liquid crystal device as claimed in claim 18wherein the breaks along the groove direction are separated by adistance which is at least double the groove pitch.
 20. A liquid crystaldevice as claimed in claim 19 wherein the breaks along the groovedirection are separated by a distance which is at least 10 μm.
 21. Aliquid crystal device as claimed in claim 1 wherein the grating is azenithal bistable surface grating.
 22. A liquid crystal device asclaimed in claim 1 wherein the grating profiles on either side of thebreak in the groove direction are substantially the same.
 23. A liquidcrystal device as claimed in claim 1 where said discontinuity is formedby a change in phase of the grating.
 24. A liquid crystal device asclaimed in claim 1 where said discontinuity is formed by a change inamplitude of the grating.
 25. A liquid crystal device comprising a layerof liquid crystal material disposed between two cell walls, at least oneregion on the internal surface of at least one cell wall comprising asurface alignment grating having a single groove direction characterisedin that the surface alignment grating comprises a plurality of breaksalong the groove direction, wherein at least one break comprises an areawhere the grooves on each side of the break are displaced relative toeach other in a direction having a component perpendicular to the groovedirection, wherein the relative displacement is substantially equal to aquarter of the groove period.